Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support for the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial
Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP) and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmit power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques and a highly efficient control signaling structure is achieved in LTE Rel. 8/9.
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGVV) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GVV). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Handover Procedure
The term Connected Mode Mobility refers to various procedures e.g. handover procedure. In particular, a 3GPP LTE handover procedure is specified in 3GPP TS 36.300: “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2”, version 11.5.0, section 10.1.2 available at http//www.3gpp.org and incorporated herein by reference. Further, details of the handover procedure relating to RRC connection reconfiguration are defined in TS 36.331: “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification)”, version 11.4.0 section 5.3.5 available at http//www.3gpp.org and incorporated herein by reference.
The Intra-E-UTRAN-Access Mobility Support for UEs in CONNECTED Mode handles all necessary steps for handover procedures, like processes that precede the final handover, HO, decision on the source network side (control and evaluation of UE and eNB measurements taking into account certain UE specific area restrictions), preparation of resources on the target network side, commanding the UE to the new radio resources and finally releasing resources on the (old) source network side.
The intra E-UTRAN handover, HO, of a UE in RRC_CONNECTED state is a UE-assisted network-controlled HO, with HO preparation signalling in E-UTRAN:                Part of the HO command comes from the target eNB and is transparently forwarded to the UE by the source eNB;        To prepare the HO, the source eNB passes all necessary information to the target eNB.        Both the source eNB and UE keep some context (e.g. C-RNTI) to enable the return of the UE in case of HO failure;        UE accesses the target cell via RACH following a contention-free procedure using a dedicated RACH preamble or following a contention-based procedure if dedicated RACH preambles are not available:        the UE uses the dedicated preamble until the handover procedure is finished (successfully or unsuccessfully);        If the RACH procedure towards the target cell is not successful within a certain time, the UE initiates radio link failure recovery using the best cell;        
The preparation and execution phase of the HO procedure is performed without EPC involvement, i.e. preparation messages are directly exchanged between the eNBs. The release of the resources at the source side during the HO completion phase is triggered by the eNB. The figure below depicts the basic handover scenario; only the first few steps are explained that are more relevant from this invention perspective:
Below a more detailed description of the intra-MME/Serving Gateway handover, HO, procedure illustrated in FIG. 3 is given where preceeding numbers refer to corresponding steps in the sequence diagram of the figure:                0 The UE context within the source eNB contains information regarding roaming restrictions which were provided either at connection establishment or at the last TA update.        1 The source eNB configures the UE measurement procedures according to the area restriction information. Measurements provided by the source eNB may assist the function controlling the UE's connection mobility.        2 A MEASUREMENT REPORT is triggered and sent to the eNB.        3 The source eNB makes decision based on MEASUREMENT REPORT and RRM information to hand off the UE.        4 The source eNB issues a HANDOVER REQUEST message to the target eNB passing necessary information to prepare the HO at the target side (UE X2 signalling context reference at source eNB, UE S1 EPC signalling context reference, target cell ID, KeNB*, RRC context including the C-RNTI of the UE in the source eNB, AS-configuration, E-RAB context and physical layer ID of the source cell+short MAC-I for possible RLF recovery). UE X2/UE S1 signalling references enable the target eNB to address the source eNB and the EPC. The E-RAB context includes necessary RNL and TNL addressing information, and QoS profiles of the E-RABs.        5 Admission Control may be performed by the target eNB dependent on the received E-RAB QoS information to increase the likelihood of a successful HO, if the resources can be granted by target eNB. The target eNB configures the required resources according to the received E-RAB QoS information and reserves a C-RNTI and optionally a RACH preamble. The AS-configuration to be used in the target cell can either be specified independently (i.e. an “establishment”) or as a delta compared to the AS-configuration used in the source cell (i.e. a “reconfiguration”).        6 The target eNB prepares HO with L1/L2 and sends the HANDOVER REQUEST ACKNOWLEDGE to the source eNB. The HANDOVER REQUEST ACKNOWLEDGE message includes a transparent container to be sent to the UE as an RRC message to perform the handover. The container includes a new C-RNTI, target eNB security algorithm identifiers for the selected security algorithms, may include a dedicated RACH preamble, and possibly some other parameters i.e. access parameters, SIBs, etc. The HANDOVER REQUEST ACKNOWLEDGE message may also include RNL/TNL information for the forwarding tunnels, if necessary.NOTE: As soon as the source eNB receives the HANDOVER REQUEST ACKNOWLEDGE, or as soon as the transmission of the handover command is initiated in the downlink, data forwarding may be initiated.        
Steps 7 to 16 provide means to avoid data loss during HO and are further detailed in 10.1.2.1.2 and 10.1.2.3.                7 The target eNB generates the RRC message to perform the handover, i.e RRCConnectionReconfiguration message including the mobilityControllnformation, to be sent by the source eNB towards the UE. The source eNB performs the necessary integrity protection and ciphering of the message. The UE receives the RRCConnectionReconfiguration message with necessary parameters (i.e. new C-RNTI, target eNB security algorithm identifiers, and optionally dedicated RACH preamble, target eNB SIBs, etc.) and is commanded by the source eNB to perform the HO. The UE does not need to delay the handover execution for delivering the HARQ/ARQ responses to source eNB.        8 The source eNB sends the SN STATUS TRANSFER message to the target eNB to convey the uplink PDCP SN receiver status and the downlink PDCP SN transmitter status of E-RABs for which PDCP status preservation applies (i.e. for RLC AM). The uplink PDCP SN receiver status includes at least the PDCP SN of the first missing UL SDU and may include a bit map of the receive status of the out of sequence UL SDUs that the UE needs to retransmit in the target cell, if there are any such SDUs. The downlink PDCP SN transmitter status indicates the next PDCP SN that the target eNB shall assign to new SDUs, not having a PDCP SN yet. The source eNB may omit sending this message if none of the E-RABs of the UE shall be treated with PDCP status preservation.        9 After receiving the RRCConnectionReconfiguration message including the mobilityControllnformation, UE performs synchronization to target eNB and accesses the target cell via RACH, following a contention-free procedure if a dedicated RACH preamble was indicated in the mobilityControllnformation, or following a contention-based procedure if no dedicated preamble was indicated. UE derives target eNB specific keys and configures the selected security algorithms to be used in the target cell.        10 The target eNB responds with UL allocation and timing advance.        11 When the UE has successfully accessed the target cell, the UE sends the RRCConnectionReconfigurationComplete message (C-RNTI) to confirm the handover, along with an uplink Buffer Status Report, whenever possible, to the target eNB to indicate that the handover procedure is completed for the UE. The target eNB verifies the C-RNTI sent in the RRCConnectionReconfigurationComplete message. The target eNB can now begin sending data to the UE.        12 The target eNB sends a PATH SWITCH REQUEST message to MME to inform that the UE has changed cell.        13 The MME sends a MODIFY BEARER REQUEST message to the Serving Gateway.        14 The Serving Gateway switches the downlink data path to the target side. The        
Serving gateway sends one or more “end marker” packets on the old path to the source eNB and then can release any U-plane/TNL resources towards the source eNB.                15 The Serving Gateway sends a MODIFY BEARER RESPONSE message to MME.        16 The MME confirms the PATH SWITCH REQUEST message with the PATH SWITCH REQUEST ACKNOWLEDGE message.        17 By sending the UE CONTEXT RELEASE message, the target eNB informs success of HO to source eNB and triggers the release of resources by the source eNB. The target eNB sends this message after the PATH SWITCH REQUEST ACKNOWLEDGE message is received from the MME.        18 Upon reception of the UE CONTEXT RELEASE message, the source eNB can release radio and C-plane related resources associated to the UE context. Any ongoing data forwarding may continue.        
When an X2 handover is used involving HeNBs and when the source HeNB is connected to a HeNB GW, a UE CONTEXT RELEASE REQUEST message including an explicit GW Context Release Indication is sent by the source HeNB, in order to indicate that the HeNB GW may release of all the resources related to the UE context.
Radio Link Failure
Radio Link Failures, RLF, have in the past been widely studied and characterized. Specifically, in the context of 3GPPP LTE, two different phases govern the behavior associated to radio link failure.
A First Phase can be characterized as follows it is starts upon detection of a radio problem; it leads to radio link failure; it corresponds to no UE-based mobility; it is distinguishable based on timer or other (e.g. counting) criteria (T1). A Second Phase can be characterized as follows: it is started upon radio link failure detection or handover failure; it leads to the UE switching to the RRC_IDLE state; it corresponds to UE-based mobility; and it is distinguishable based on timer (T2).
In the table below, it is described how mobility is handled with respect to radio link failure.
CasesFirst PhaseSecond PhaseT2 expiredUE returns to the sameContinue as if noActivity is resumed byGo via RRC_IDLEcellradio problemsmeans of explicit signalingoccurredbetween UE and eNBUE selects a differentN/AActivity is resumed byGo via RRC_IDLEcell from the same eNBmeans of explicit signalingbetween UE and eNBUE selects a cell of aN/AActivity is resumed byGo via RRC_IDLEprepared eNB (NOTE)means of explicit signalingbetween UE and eNBUE selects a cell of aN/AGo via RRC_IDLEGo via RRC_IDLEdifferent eNB that is not prepared (NOTE)NOTE:a prepared eNB is an eNB which has admitted the UE during an earlier executed HO preparation phase.
In the Second Phase, in order to resume activity and avoid going via RRC_IDLE when the UE returns to the same cell or when the UE selects a different cell from the same eNB, or when the UE selects a cell from a different eNB, the following procedure applies:                The UE stays in RRC_CONNECTED;        The UE accesses the cell through the random access procedure;        The UE identifier used in the random access procedure for contention resolution (i.e. C-RNTI of the UE in the cell where the RLF occurred+physical layer identity of that cell+short MAC-I based on the keys of that cell) is used by the selected eNB to authenticate the UE and check whether it has a context stored for that UE:        If the eNB finds a context that matches the identity of the UE, it indicates to the UE that its connection can be resumed;        If the context is not found, RRC connection is released and UE initiates procedure to establish new RRC connection. In this case UE is required to go via RRC_IDLE.        
The radio link failure procedure applies also for RNs, with the exception that the RN is limited to select a cell from its DeNB cell list. Upon detecting radio link failure, the RN discards any current RN subframe configuration (for communication with its DeNB), enabling the RN to perform normal contention-based RACH as part of the re-establishment. Upon successful re-establishment, an RN subframe configuration can be configured again using the RN reconfiguration procedure.
If the recovery attempt in the second phase fails, the details of the RN behavior in RRC_IDLE to recover an RRC connection are up to the RN implementation.
Radio Link Failure is described in detail in 3GPP TS 36.300: “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2”, version 11.5.0, section 10.1.6 available at http//www.3gpp.org and incorporated herein by reference.
Technology Areas
This invention can be used as a solution for improving mobility robustness in many technologies including:
Heterogeneous Networks
A Heterogeneous network is generally referred in this invention for deployment scenarios where the neighboring cells differ (sometimes substantially) in transmit power/cell range/size. The bigger cell/eNB (in transmit power/cell range/size) is generally referred to as Macro (or Master, Main, Aggressor etc.) and the small cell/eNB is referred to as Small (or Pico, Secondary, Victim etc.). This invention further does not distinguishes within e.g. Small Cells/eNBs i.e. Pico and Small eNB are treated/named in a similar way irrespective of their definition (and difference in real transmit power/size) elsewhere—this invention applies for all scenarios with mixed cell/eNB deployments.
Small Cells
Explosive demands for mobile data are driving changes in how mobile operators will need to respond to the challenging requirements of higher capacity and improved Quality of user Experience (QoE). Currently, fourth generation wireless access systems using Long Term Evolution (LTE) are being deployed by many operators worldwide in order to offer faster access with lower latency and more efficiency than 3G/3.5G system. Nevertheless, the anticipated future traffic growth is so tremendous that there is a vastly increased need for further network densification to handle the capacity requirements, particularly in high traffic areas (hot spot areas) that generate the highest volume of traffic. Network densification—increasing the number of network nodes, thereby bringing them physically closer to the user terminals—is a key to improving traffic capacity and extending the achievable user-data rates of a wireless communication system.
In addition to straightforward densification of a macro deployment, network densification can be achieved by the deployment of complementary low-power nodes respectively small cells under the coverage of an existing macro-node layer. In such a heterogeneous deployment, the low-power nodes provide very high traffic capacity and very high user throughput locally, for example in indoor and outdoor hotspot positions. Meanwhile, the macro layer ensures service availability and QoE over the entire coverage area. In other words, the layer containing the low-power nodes can also be referred to as providing local-area access, in contrast to the wide-area-covering macro layer.
The installation of low-power nodes respectively small cells as well as heterogeneous deployments has been possible since the first release of LTE. In this regard, a number of solutions have been specified in recent releases of LTE (i.e., Release-10/11). More specifically, these releases introduced additional tools to handle inter-layer interference in heterogeneous deployments. In order to further optimize performance and provide cost/energy-efficient operation, small cells require further enhancements and in many cases need to interact with or complement existing macro cells. Such solutions will be investigated during the further evolution of LTE—Release 12 and beyond. In particular further enhancements related to low-power nodes and heterogeneous deployments will be considered under the umbrella of the new Rel-12 study item (SI) “Study on Small Cell Enhancements for E-UTRA and E-UTRAN”. Some of these activities will focus on achieving an even higher degree of interworking between the macro and low-power layers, including different forms of macro assistance to the low-power layer and dual-layer connectivity. Dual connectivity implies that the device has simultaneous connections to both macro and low-power layers.
Some deployment scenarios assumed in this study item on small cell enhancements will be discussed below. In the following scenarios, the backhaul technologies categorized as non-ideal backhaul in TR 36.932 are assumed.
Both ideal backhaul (i.e., very high throughput and very low latency backhaul such as dedicated point-to-point connection using optical fiber) and non-ideal backhaul (i.e., typical backhaul widely used in the market such as xDSL, microwave, and other backhauls like relaying) should be studied. The performance-cost trade-off should be taken into account.
A categorization of non-ideal backhaul based on operator inputs is listed in the table below:
BackhaulLatencyPriorityTechnology(One way)Throughput(1 is the highest)Fiber Access 110-30ms10 M-10 Gbps1Fiber Access 25-10ms100-1000 Mbps2Fiber Access 32-5ms50 M-10 Gbps1DSL Access15-60ms 10-100 Mbps1Cable25-35ms 10-100 Mbps2Wireless Backhaul5-35ms10 Mbps-100 Mbps1typical, maybe up to Gbps range
Fiber access which can be used to deploy Remote Radio Heads (RRHs) is not assumed in this study. HeNBs are not precluded, but not distinguished from Pico eNBs in terms of deployment scenarios and challenges even though the transmission power of HeNBs is lower than that of Pico eNBs. The following 3 scenarios are considered.
Scenario #1 is illustrated in FIG. 5 and is the deployment scenario where macro and small cells on the same carrier frequency (intra-frequency) are connected via a non-ideal backhaul. User are distributed both for outdoor and indoor.
Scenario #2 is illustrated in FIGS. 6 and 7 and refers to a deployment scenario where macro and small cells on different carrier frequencies (inter-frequency) are connected via a non-ideal backhaul. User are distributed both for outdoor and indoor. There are essentially two different scenarios #2, referred herein as 2a and 2b, the difference being that in scenario 2b an indoor small cell deployment is considered.
Scenario #3 is illustrated in FIG. 8 and refers to a deployment scenario where only small cells on one or more carrier frequencies are connected via a non-ideal backhaul link.
Depending on the deployment scenario, different challenges/problems exist which need to be further investigated. During the study item phase such challenges have been identified for the corresponding deployment scenarios and captured in TS 36.842; more details on those challenges/problems can be found there.
In order to resolve the identified challenges which are described in section 5 of TS36.842, the following design goals are taken into account for this study in addition to the requirements specified in TR 36.932.
In terms of mobility robustness:                For UEs in RRC_CONNECTED, Mobility performance achieved by small cell deployments should be comparable with that of a macro-only network.        In terms of increased signaling load due to frequent handover:        Any new solutions should not result in excessive increase of signaling load towards the Core Network. However, additional signaling and user plane traffic load caused by small cell enhancements should also be taken into account.        In terms of improving per-user throughput and system capacity:        Utilizing radio resources across macro and small cells in order to achieve per-user throughput and system capacity similar to ideal backhaul deployments while taking into account QoS requirements should be targeted.Dual Connectivity        
One promising solution to the problems which are currently under discussion in 3GPP RAN working groups is the so-called “dual connectivity” concept. The term “dual connectivity” is used to refer to an operation where a given UE consumes radio resources provided by at least two different network nodes connected with a non-ideal backhaul. Essentially, the UE is connected with both a macro cell (macro eNB) and small cell (secondary or small eNB). Furthermore, each eNB involved in dual connectivity for a UE may assume different roles. Those roles do not necessarily depend on the eNB's power class and can vary among UEs.
Since the study Item is currently at a very early stage, details on the dual connectivity are not decided yet. For example the architecture has not been agreed on yet. Therefore, many issues/details, e.g. protocol enhancements, are still open currently. FIG. 9 shows an exemplary architecture for dual connectivity. It should be only understood as one potential option; the invention is not limited to this specific network/protocol architecture but can be applied generally. The following assumptions on the architecture are made here:                Per bearer level decision where to serve each packet, C/U plane split        As an example UE RRC signaling and high QoS data such as VoLTE can be served by the Macro cell, while best effort data is offloaded to the small cell.        No coupling between bearers, so no common PDCP or RLC required between the Macro cell and small cell        Looser coordination between RAN nodes        SeNB has no connection to S-GW, i.e. packets are forwarded by MeNB        Small Cell is transparent to CN.        
Regarding the last two bullet points, it should be noted that it's also possible that SeNB is connected directly with the S-GW, i.e. S1-U is between S-GW and SeNB. Essentially there are three different options w.r.t the bearer mapping/splitting:                Option 1: S1-U also terminates in SeNB; depicted in FIG. 10a         Option 2: S1-U terminates in MeNB, no bearer split in RAN; depicted in FIG. 10b         Option 3: S1-U terminates in MeNB, bearer split in RAN; depicted in FIG. 10c         
FIG. 10a-c depict those three options taking the downlink direction for the U-Plane data as an example. For explanation purpose, option 2 is mainly assumed for this application, and is the basis for FIG. 9 too.
One of the main aims in SCE (Small Cell Enhancement) study item and generally in Heterogeneous deployment is that the offloading of UEs to the small/Pico Cells are maximized which means that more and more UEs are offloaded to small/Pico Cells as well as the time that the UEs stay in the small/ Pico Cells (before their connection moving out to a Macro Cells) is maximized i.e. the ToS (Time of Stay) maximization is an important aim in the Small Cell Enhancement and generally in Heterogeneous deployment.
Shortcomings with Heterogeneous Deployment
A state where the UE is known to the network at the Access Stratum level is generally referred to as Connected Mode. In this respect, Connected Mode mobility typically indicates when the UE moves from one location to the other such that the signal condition in the source area is getting weaker and better in the destination area. The Connected Mode mobility is realized by means of Handovers. In Heterogeneous deployment scenarios the source/destination cells are typically of different size/transmit power.
It has been widely acknowledged that Connected Mode Mobility in Heterogeneous deployment scenarios is not as robust as desired and that handover failures, HOF, or radio link failures RLF have recently been put under technical review; for a detailed description it is refer to 3GPP TR 36.839: “Evolved Universal Terrestrial Radio Access (E-UTRA); Mobility enhancements in heterogeneous networks” version 11.1.0, sections 5.2 and 5.4 available at http//www.3gpp.org and incorporated herein by reference.
The mobility is especially weak when the UE moves out of the Small/Pico cell towards the Macro cell. This is mainly so since the radio condition in the Small/Pico cell's edge is weaker and interfered by the Macro cells transmissions. In these situations it is difficult to successfully receive the handover command by the UE.
Some kind of preventive solutions do exist. The preventive solutions in the state-of-the-art are quite complex and in some cases not completely clear how these will work; for example:
Repetition of HO CMD message by cells other than the source cell (RRC Diversity) e.g. the handover target cell requires that the UE be able to receive from 2 different cells [Simultaneous or One by one] and it will be unattractive and complex since:                Simultaneous reception requires dual connection capability which may not be available with all UEs and dual connectivity itself may not be a desirable solution in some deployment scenarios (e.g. in Scenario 1 of SCE).        One by one (first by source and then by target): In this case, it is not clear when and how will the UE know, which C-RNTI to use, which SRB configuration to use etc. when trying to receive the HO CMD from the Target Cell.        
Requiring multiple (interfering) neighbor cells to coordinate/blank their transmission when the HO CMD is to be sent/received to/by the UE. However, this alone is not sufficient for non ideal ABS coordination among macro cells and larger CRE bias cases.
One of the ways to achieve “prevention” is by sending the HO CMD early by the source cell (while the Radio Conditions are sufficiently good); this suffers from:                Minimizing offloading gains (the UE should stay on the small cell for as long as possible)        Un-necessary handovers/mobility/signaling        More HO interruption        
Other possible line of solution for improving the Mobility Robustness in heterogeneous deployment could be curative in nature. For example, if the mobility/ handover fails (e.g. HOF, RLF happens) then how to minimize the damages (e.g. by faster reestablishments). The curative methods are not sufficient since they lead to more complexity (in enhancing Re-establishment) and still cause some jitters.
Another problem in Heterogeneous deployment is the UE battery consumption in:    1) Finding/discovering the small cell layer since these small cells may not be ubiquitously present everywhere.    2) Measuring 2 source frequencies while in dual connectivity which might be un-necessary sometimes.
Improving UE battery life in Heterogeneous deployment is referred to as the second problem in the remaining text.